2-methylchlorophenoxyacetate herbicides



Patented May 5, 1953 UNITED STATES OFFICE 2-METHYLCHLOROPHENOXYACETATEHEa IcIn s Frank lialbot and Reginald Thomas, Foster, Widnes, England,ass'ignors to ImperialChemical Industries Limited, a corporation ofGreat Britain No Drawing. Application Marclil3, 1950, Serial No.149,456. In Great BritainMar'ch 28,1949 7 This invention relates to theproductionof organic compounds and especially to an improved process forthe production of soluble salts of the 4-chloroand 35% of theG-chloro-isomer, is

reacted as such or Whether it is separated into its constituents. Intheformer case, preferential reaction'of the i-chloro isomer takes placeand the resultant methylchloro-phenoxyacetates' are composed'of '70% ofthe active 2:4 isomer and 30% of the relatively inactive 2:6 isomer. Thetwo isomeric chlorocresols may alternatively be completely separated,for instance by vacuum distillation, or they may be partially separatedby distilling off most of the lower-boiling 2:6 compound leaving asresidue all the 214 material and a small amount of the 2:6 compound, the2:4 isomer or its concentrate being then condensed in the usual way withmonochloroacetic acid. When such separation is resorted to, the finalmethylchlorophenoxyacetic acid salt consists principally of the activeisomer but on the other hand neither the totalyield from the reactionnor the yield of active mate rial is as good as that obtained from thecon densation of the crude chlorocresol mixture.

Whichever of these methods is used the crude liquid'product contains insolution in addition to the sodium salt of 2-methylchlorophenoxyace ticacid, other salts formed during the reaction, such as sodium glycollateand sodium chloride, and the presence of these impurities causesseparation from the cooled solution of crystals of sodiummethylchlorophenoxyacetates and other solids when the concentration ofthe desired or-' ganic salts is greater than about 15% by weight, i. e.about by weight of the active 2:4 derivative in the case Where crudechlorocresol is used. This precipitation in the more concentratedsolutions leads to obvious" transport d'ifliculties, and

it' has previously been the practice to use the 9 Claims.- (01.7192.4-5'

up solutions when and where they are req The total solids present in thecrude liquid condensation product are composed of about 57% ctsodium-2-methylchlorophenoxyacetate, 24% of sodium chloride and 10% ofsodium glycollate,

the remainder consisting of sodium cresylate and other minor impurities.Complete removal of the water, therefore, will give a solid productwhose content of methylchlorophenoxyacetic acid salts cannot exceed 57%,and further purification has so far resulted in loss of yield. As'already mentioned, these salts can be precipitated'by cooling their moreconcentrated solutions; but such separation is only partial. It istherefore a further object of the invention to obtain a solid productcontaining substantially all the sodium methylchlorophenoxyacetate inhighcon ceiitration.

In'Bri-ti'sh Patent No. 623,217 there is described and olaimed' acontinuous method for obtaining solid 2-methyli-chlorophenoxyacetic acidcon taining not more than 10% of water and in a suitably granular form,from the crude liquidreaction product obtained from the condensation of4-chloro-o-cresol, alone or admixed with other chloro-cresols, withmonochloroacetic' acid in the presence of aqueous alkali.

In this process the crude product is continu-' ously acidified at atemperature at which the precipitated crude2-methyl-4chlorophenoxyacetic acid is molten while at the same themixture is subjected to vigorous agitation, the mixed aqueous and moltenorganic phases are continuously fed to a second vesselmaintained at atemperature at which the 2-methyl-4-chlorophenoxyacetic acid'remainsmolten, and the'twoph'a'ses' are allowed to'separate by settling, thelayer of molten 2-methyli-chloro-phenoxyacetic acid being run oii andallowed to cool. The crude product obtained by reactingthe crude mix-'ture of mono'chlorinated o-cresols with monochloroactic acid, asdescribed in British Patent No. 573,479, can also be treated accordingto the saidprocess, the acid finally obtained being in this case amixture of the 2:4- and 2:6-isorners in the relative proportions of to30 parts by weight.

We have now found that the Z-methylchlorophenoxyacetic acids such as areproduced in the aforementioned process are extremely suitable materialsfrom which to prepare the water-solu-' uired.

above the melting point ofzthe said acid and separating the two phases,and passing the sepa-' rated molten acid into a third vessel into whichis also introduced an alkaline reagent which will react with the acid togivea water-soluble salt, advantageously in such amount that an excessof approximately 0.5% to 1.7% over the theoreti cal is present in theproduct, together with, if desired, sufficient water to produce aconcentrated solution of the said salt. Advantageous ly, the variousoperations are carried out continuously.

By the term highly concentrated form we mean either a concentratedaqueous solution or, a. solid containing over 65% of the salts of themethylchlorophenoxyacetic acids. e

In the descriptive matter which. follows the liquid product referred tois in all cases that obtained by the condensation of monochloroaceticacid with a crude monochlorinated ocresol, but it is to be understoodthat the liquid products obtained by use of the alternative methods ofpreparation indicated ,hereinbefore may also be treated according to thepresent invention. H

A saturated solution of the sodium salt of 2,-methyl-4-chlorophenoxyacetic acid will contain about 40% of the salt,while the 2-methyl-6- chlor-o-derivative is rather more soluble. By useof the process of the invention. the principal impurities present in theoriginal solution are removed in the aqueous phase separated in thesecond stage and it is thus possible. to obtain stable solutions of thedesiredsalts up to the limit of saturation, or alternatively to obtain asolid material containing a high proportion of these salts. Suchsolutions will contain between 36% and 50% of the salts and only veryminor proportions of impurities and they are stable at normaltemperatures, no precipitation of solid taking place on standing. Thechlorocresol, obtained from the sodium chlorocresylate afteracidification, is however mostly .retained in the molten acid layer.This impurity may be removed by other methods, for example, by

first subjecting the partly acidified liquid condensation product tosteam distillation as described in British Patent No. 592,827,; and itis desirable to carry out this step on the crude liquid product beforesubjectingthe latter to the process of the invention.

The alkaline reagent is suitably or carbonate of an alkali metal or ofammonium, the sodium compounds being preferred on account of theircheapness and ready availability. Any hydroxide or carbonate which isnot decomposed at the reaction temperature and whose cation gives awater-soluble salt of 2- the rant water while .in addition water isformed as a product of the neutralisation reaction. Both these factorsmust therefore be taken into account when estimating the total waterrequired. If caustic soda is used, for example, it is found that anaqueous solution containing approximately 20% by weight of caustic sodawill give a practically saturated solution of the sodium2methylchlorophenoxyacetates, that is, one having a content of 8334% ofthe active isomer.

This satLu'ated solution is stable at normal temperatures and can bestored with little deposition of solid, but we have found that solutionsof rather lower concentration containing about 30% of the active saltalthough equally stable at normal temperatures possess a greaterstability at low temperatures and can be cooled to temperatures down to10 0. without any loss of homogeneity. When a concentrated so,- lutionis required, therefore, the alkaline reagent is advantageously anaqueous solution of strength sufficient to give a product whose contentof the active z-methyll-chlorophenoxyacetic acid salt is about 30%. Forpractical purposes, the product obtained from 0% caustic sodamay bediluted to give this solution of optimum stability. On the other hand,the latter may be directly obtained by using a less concentratedsolution of caustic soda, 16%, by weight giving a solution of therequired content of the active isomer. Alkali metal carbonates can beemployed instead of the'hydroxides, but evolution of carbon dioxideisliable to cause some difiiculty and, in the case of sodium carbonate,there is the additional disadvantage that its solubility in water is toolow to enable a 30% active solution of the required salts to be directlyobtained.

If the solid salts are required, these may be simply obtained byreacting the molten acids either with solid alkali or with a highlyconcentrated aqueous solution of the alkali which is liquid at thereaction temperature suchas, for example, a solution of caustic soda ofup to 70% by weight. The latter method is in general more convenient inoperation since it is easier thus to obtain an intimate mixture of thereactants and to avoid the formation of pockets of unreacted material.Solids containing up to 65% of active material may be obtained in thisway. Phenolphthalein may be used as indicator to the alkalinity of theproduct, but it is preferable in the preparation of the solid salts touse slightly more than the theoretical amount of alkali, a productcontaining about 5% excess being more easily handled and more suitablefor incorporation in herbicidal compositions.

The stability of the concentrated solutions of salts of,2-methylchlorophenoxyacetic acid is critically affected by the quantityof free alkali present. For instance, solutions containing about 30% ofthe active salt must also contain from about 0.5% to about 1.7% of freealkali. If the alkali content is less than 0.5% the solutions will notremain clear on dilution, while an alkali content of much over 1.7% hasa saltingout effect and crystallisation of the salt will occur. Theme ofphenolphthalein as indicator during the neutralisation step ensures thatthe solution contains the minimum amount of free alkali, and the alkaliaddition is carefully regulated so that its content in'the product willnot exceed the above critical value.

It is therefore advantageous to have present in admixture with the saltof 2-methyl-4-chloroph'enoxyacetic acid, whetherthe latter be in solidform or in solution, free alkali in amount approximately equalto from1.8% to 6% ofthe weight of the said salt. T

The crude liquid consensation product used as raw material in thepresent process contains from 5% to of the sodium saltof,2'-'methylchlorophenoxyacetic acid. More concentrated solutions donot give entirely satisfactory results" since on acidification with amineral acid the sodium sulphate or sodium chloride formed are alsoprecipitated with the organic acid at the higher concentrations, and inaddition the differ-' ence between their specific gravities and that ofthe free molten acids becomes too smallto ensure a satisfactoryseparation;

The acidification step may be carried out by means of aqueoushydrochloric or sulphuric acids. The strength of acid is not criticalbut it are just acid to Congo, red, a pH of. about 3'thus beingmaintained.

In the neutralisation vessel, a pH of about 1 1 is desirably maintainedand the solution of alkali should be fed at such a rate that thereaction mixture is alkaline to phenolphthalein.

In one form of the invention the crude condensation product of amonochlorinated o-cresol and monochloroacetic acid diluted to aconcentration of 10% of sodium 2-rnethy1-4 chlorophenoxy acetate istreated according. to the process of British Patent No. 623,217 bysubjecting it to con-' tinuous acidification with concentrated aqueoushydrochloric acid at a temperature between 80 and C., that is, above themelting point of the" mixed methylchlorophenoxyacetic acids'qthe mixturebeing vigorously stirred throughout the operation, running off theproduct continuously into a second vessel also heated to a temperaturehigh enough to maintain the acids ina molten state and allowing tosettle. arated from the molten organic phase and the latter iscontinuously passed into a third stirred and heated vessel into whichthere is simultaneously fed an aqueous solution of caustic alkali,

the strength of the latter being preferably chosen r so that theresultant solution of methylchlorophenoxyacetates contains at leastabout 30% 'by weight of the active isomer. The alkali feed is at such arate that the product is kept just a1- kaline to phenolphthalein andspecific gravity determinations are carried out at frequent intervals inorder to provide a rough guide to the strength of the product andconsequently to the operation of this stage. The liquid product from theneutralisation vessel is cooled and passed directly to collectingvessels, the contents of each of which are analysed for theirconcentration of active material by the standard method. They are thendiluted with water if necessary to bring them to a concentration of 30%active salt and are passed directly to storage.

Should the solid salts of the methylchlorophenoxyacetic acids berequired, these may be readily obtained by a modification of the aboveprocess in which the separated molten acids are reacted The aqueousphase is sep- 6 either with solid alkali or with a ,hot concen tratedsolution thereof.

In the first case, solid alkali may be added with vigorous agitation tothe molten acids prepared as before and maintained at a" temperature ofabout C. until the mixture contains a slight excess of alkali,- afterwhich it is advanta'geously heated gently for a short period in order toremove any retained water produced in the reaction. It may then becooled and ground to a suitable texture. This method requires a highlyefficient mixer since otherwise unreacted material is liable to beretained within the mass.

In afurther form of the invention; ahot conce'ntra'ted solution ofalkali is used in place of the solid. The molten mixture of organicacids prepared as described above is fedinto the stirred and" heatedneutralisation vessel simultaneously with a hot concentrated solution ofalkali which is liquid at the temperature ofthe molten acids, that is,at about 80 C. For instance, a solution of caustic soda containing from60% to 70% of NaOH may be employed. As before, the alkali is added atleast until the mixture shows" alkaline reaction to phenolphthalein, butit is again found desirable in practice to have a slight excess ofalkali inthe final product. The fluid mass is Well mixed and itstemperature is then ra'i'sed' to 110 C. in order to remove most of thewater present. A product containing as much as" 10% of water willsolidify and is capable of being finely'ground, but this quantity ofwater is generally initially present in the molten acids and afterreaction the fluid mass will also contain the water whichwas present inthe solution together with the water formed during the reaction.

After removal of the excess water the product is cooled, forming abrittle solid which may be ground to a free-fiowingpowder readilysoluble in water. This will contain 80% or moreoremethylchlorophenoxyacetates with a content of at least 56% of activematerial.

The melting point of the mixed acids is about 70 C. and the contents ofthe acidification vessel and separator should be maintained at a temperature between 80" and C When a solid prod,- uct is requiredthe contentsof the neutralisation vessel are preferably kept at as high a tern.-peiature as possible, say up to C'., in order to drive off the excesswater, the heat for this purpose being obtained partially fromtheheatliberated by the neutralisation process and partially fromexternal heating means. In the preparation of the concentrated solution,however, the temperature of this vessel should be kept fairly low, saynot higher than 70 (3., in order to. prevent loss of water byevaporation. The vessel is' then suitably fitted with cooling meanssuchas for example a water jacket.

The following examples illustrate but do not limit the invention. 1

Example 1 The starting material was a crude solution of the mixed sodiumsalts of 2methyl-4- ch1oroand 2 methyl-6-chloro-phenoxyacetic acidscontaining 10% or" the former. This solution was contained in a gravitytank and was passed through a steam-jacketed preheater to theacidification vessel simultaneously with 36% hydrochloric acid. The twoliquids were allowed to enter the vessel at rates of 380 litres and 42litres per hour re-' spectively.

The acidification vessel was an enclosed steamjacketed lead-lined:vessel fitted with a flat paddle accuse:

stirrenand a 'thermometer. pocketand having two dip pipes for theentrance ofthe 10% solution and the .hydrochloric acid together with anoverflow pipe near the top through which the reaction mixture passed .tothe separator. The contents of the acidiflcationvesselwere heatedtoy85C. with stirring, and the suspension of molten 2-methylchlorophenoxyacetic acids in aqueous salt solution overfiowedcontinuously into the steamjacketed separation vessel,,where it wasallowed to settle. An overflow pipeserved to remove the aqueous layer;whilea run-off pipe conveyed the separated molten, .acids- :to theneutralisation vessel: The. latter was-fitted with a water jacket andstirrer and the usual feed pipes for the molten acids and the alkalisolution and an overflow pipe tocarryoff the product. 16% caustic sodasolution was run in at the rate of 51 litres per hour and the mixturewell stirred. When the product was passing over into the collectingvessel, cool- 96%. The following table gives the analytical figuresforthe starting material and the product.

Percent Percent Percent Sample active sodium sodium 2.4-salt chlorideglycollate Feed stock 10.06 6.1 2.7 Product 30.00 0.6 1.3

After 8 hours storage of the product at 10 C. no precipitation tookplace.

Example 2 The starting material was the same as in Example 1, the moltenacids being obtained as before by acidification and separation. In thiscase, however, before entering the main acidification vessel the hotcrude solution was passed through a subsidiary vessel into which wassimultaneously fed sufficient 36% hydrochloric acid to set free thechlorocresol from the dissolved sodium chlorocresylate, and thischlorocresol was removed by passing live steam into the mixture.

After acidification and separation of the molten acids as before, thesewere conveyed to the neutralisation vessel which in this case wassurrounded by a steam jacket. 63% caustic soda solution maintained at 80C. was run in at the rate of 11.8 litres per hour (measured at 80 C.)and the mixture well stirred. The contents of the neutralisation vesselwere maintained at a temperature of about 110 C. by means of the heat ofreaction and steam heating when necessary and molten sodium2-methylchlorophenoxyacetate was run off continuously, via the overflowpipe, and cooled to give a solid containing about 63% of sodium2-methyl-4-chlorophenoxyacetate. The total yield of active material wasabout 96% of that fed to the process.

Having now particularly described and ascertained the nature of ourinvention and in what manner the same is to be performed, we declarefrom the crude liquid product obtained by the condensation of amonochlorinated o-cresol and monochloroaceticv acid in aqueous, alkalinesolution, said process comprising the steps of acidifying said crudeliquid product while vigorously agitating the same in a first vesselmaintained at.

a temperature at which the precipitated 2-methyl chlorophenoxyaceticacids are molten, feeding the resulting mixed aqueous and molten organicacid phases into a second vessel maintained at a temperature above themelting point of said acids and separating the two phases, andthereafter passing the separated molten organic acid phase into a thirdstirred vessel and treating said acid phase with an alkaline reagentwhich reacts with the acid to give a water-soluble salt, said alkalinereagent being employed in an amount sufficient to provide an excess ofapproximately 0.5% to 1.7% over that theoretically required for the saltformation.

2. A process according to claim 1 in which the crude liquid containsfrom 5% to 20% by weight of the sodium salt of2-methylchlorophenoxyacetic acid.

3. A process according to claim 1 in which the alkaline reagent is addedat such a rate that the pH of the reaction mixture is maintained atabout 4. A process according to claim 1 in which the alkaline reagent iscaustic soda together with from 0% to approximately 84% of its weight ofwater.

5. A continuous process according to claim 4 in which the caustic sodasolution contains approximately 84% of water.

6. A continuous process according to claim 4 in which the caustic sodasolution contains from 30% to 40% by weight of water, and excess wateris removed from the product.

'7. The process of claim 1, wherein said alkaline reagent is added withsufficient water to produce an aqueous solution containing at least 25%by weight of the salt of 2-methyl-4-chlorophenoxyacetic acid.

8. A composition of matter containing a 2- methylchlorophenoxyacetatewhich is capable of giving a physically stable concentrated solutionupon being dissolved in water, said composition comprising at least 65%by weight of a watersoluhle salt of 2-methyl-4-chlorophenoxyacetic acidtogether with free alkali in amount equal to from 1.8% to 6% of theweight of the said salt.

9. An aqueous solution of a Z-methylchlorophenoxyacetate comprising atleast 25% by weight of a water-soluble salt of 2-methyl-4-chlorophenoxyacetic acid and from 1.8% to 6% by weight of said salt offree alkali, said free alkali stabilizing said solution againstprecipitation of said salt.

FRANK TALBOT. REGINALD THOMAS FOSTER.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,485,146 Foster Oct. 18, 1949 2,511,784 ONeal June 13, 1950FOREIGN PATENTS Number Country Date 573,476 Great Britain Nov. 22, 1945573,479 Great Britain Nov. 22, 1945 573,510 Great Britain Nov. 23, 1945592,827 Great Britain Sept. 30, 1947

8. A COMPOSITION OF MATTER CONTAINING A 2METHYLCHLOROPHENOXYACETATEWHICH IS CAPABLE OF GIVING A PHYSICALLY STABLE CONCENTRATED SOLUTIONUPON BEING DISSOLVED IN WATER, SAID COMPOSITION COMPRISING AT LEAST 65%BY WEIGHT OF A WATERSOLUBLE SALT OF 2-METHYL-4-CHLOROPHENOXYACETIC ACIDTOGETHER WITH FREE ALKALI IN AMOUNT EQUAL TO FORM 1.8% TO 6% OF THEWEIGHT OF THE SAID SALT.